1. Field of the Invention
This invention relates to local area networks and particularly to a wireless local area network having distributed control and supporting roaming of mobile units in terms of their contact with network access points.
2. Description Of The Prior Art
Xircom, Inc. has developed wireless local area network computer communication devices designated "Netwave" and described in published patent document PCT/US94/07004 entitled "Virtual Carrier Detection for Wireless . . . " inventors Kenneth J. Biba et al., published Jan. 5, 1995, incorporated herein by reference. The Netwave technology uses 2.4 GHz ISM band frequency hopping radio data transmissions. This technology allows users to form computer networks of mobile computers without the need to have these mobile computers connected by wires.
Although the Xircom Netwave product is a local area network, it achieves maximum potential when used in the form of multiple connected network service areas that provide coverage throughout a user's facility (e.g. an office building or group of buildings). This is analogous to a cellular radio telephone system in which a total coverage area is divided up into cells that have intelligent relay stations. The term "roaming" comes from cellular telephony and, in that context, describes the process by which a mobile telephone user moves from cell to cell while maintaining the virtual telephone line connection. In the case of the Netwave product, roaming provides the same benefit to the user; however, because the Netwave product supports a packet data network, the underlying roaming process differs.
To provide extended coverage of an area, Netwave uses a dedicated station called an Access Point (AP). An AP is usually connected to some form of fixed "backbone" network (i.e. wired, fiber or point-to-point radio) and it is a relay station that receives radio data packets off the air and transmits them on the fixed backbone and vice versa. FIG. 1 shows a system with four Access Points 10, 12, 14, 16 connected to a wired backbone network 20 that includes resources 24 such as printers and file servers.
Each AP 10, 12, 14, 16 is surrounded by a boundary indicating the edge of the region covered by its radio transceiver. This region is called herein the Basic Service Area (BSA). When APs are linked on backbone network 20, these BSAs can be joined to form an Extended Service Area (ESA). Roaming enables the Netwave user, by using a Mobile Unit (MU) (not shown), to treat the ESA as if it were only one BSA.
The above-referenced patent disclosure describes in detail a method and apparatus for roaming, beginning at page 48. However, it has been found that an improvement is possible over the roaming technique described therein. There is needed a more reliable method for establishing, by a well defined process, with which AP a particular mobile unit should be associated at any given time and when and under what conditions the mobile unit should change its associated AP.
Other wireless local area networks faced with the roaming problem typically use as a measure of goodness of connection quality between a particular mobile unit and an access point, the RSSI (received signal strength indicator) measurement, which is an RF measurement of carrier strength only. For instance, in cellular telephony a cellular site monitors the RSSI of a particular cellular telephone and in conjunction with other cellular sites triangulates to determine the location of that particular cellular telephone, so as to determine with which site that cellular telephone should be linked. Other wireless local area networks use RSSI measurement in the mobile unit to determine which is the closest access point.
It has been found by the present inventors that the RSSI technique, whether carried out by the access point or the mobile unit, is inadequate because it only measures the radio signal strength and does not adequately measure optimum communications quality.
Thus for effective roaming in a local area network, a better approach to roaming and determining the optimum association between a mobile unit and access points is needed.